Till recently, just two classes of antiretroviral drugs, reverse transcriptase inhibitors and protease inhibitors, were on the market to treat HIV infection. Last week, the FDA approved Merck's raltegravir (Isentress), which interferes with viral replication at a different point, blocking the enzyme integrase to prevent the integration of the viral genetic material into host DNA.

In our September issue, The Scientist reported on Pfizer's maraviroc, approved in the US in August under the brand name Selzentry. While other HIV drugs work inside the cell, maraviroc blocks the activity of CCR5, the coreceptor on the cell surface that facilities HIV's entry into the cell.

The two are the first compounds with a novel mechanism of action against the virus to be approved in several years. So far, both drugs are approved for use only in patients who have developed a resistance to other drugs.